The Earth's curst under western India is unusually hot and is in 'a state of continuous uplift', according to geologists of the National Geophysical Research Institute (NGRI) in Hyderabad. They believe this discovery may help explain the occurrence of devastating earthquakes in the peninsular India witnessed in recent years.
Major earthquakes in the Indian subcontinent are the result of stresses built up due to the collision of the Indian plate with the Asian plate that gave rise to the Himalayas. These earthquakes had taken place along the plate boundary. The peninsular shield is supposed to be relatively seismically quiet. However it has witnessed several damaging earthquakes in Koyna (1967), Latur-Killari (1993), Jabalpur (1997) and Bhuj (2001). Despite a large number of investigations, the cause of their occurrence is unclear. Some scientists believe that Koyna earthquakes are induced by a nearby reservoir while others believe them to be of tectonic origin.
Now a detailed study including the characterization of the rocks sandwiched between the 'Deccan Traps' and the earth's crust in Latur and Koyna regions has shed new light on the origin of 'intraplate' earthquakes, the NGRI scientists report1.
The Deccan Traps refer to the pile of volcanic rocks, 1 to 2 kilometres thick, that covered nearly half a million square kilometre area of central and western India following one of the largest eruptions on Earth 65 million years ago. What underlies this volcanic rock cover had remained an enigma and the NGRI team set out to crack it in the hope of looking for clues for earthquakes in the peninsular India.
"It has been assumed all these years that these volcanics are resting over granitic-gneissic crust but our study has overthrown this notion," Gopalakrishnarao Parthasarathy of NGRI, one of the authors of the paper told Nature India. Granitic gneiss (pronounced 'nise') is an igneous rock and is formed from water-rich magma. But the NGRI findings indicate that the basement rock is not granitic but 'granulite' (a deeper rock) with very low water content and rich in carbondioxide (CO2) containing as much as two grams of CO2 for every 100 grams of the mineral grains. Granulites are formed at temperatures around 700°C and pressure range 6-12 kilobars corresponding to a depth of 30km in the earth's interior.
The NGRI study was an attempt to examine and compare the prevailing geologic and tectonic structure of Latur and Koyna regions by analyzing active and passive seismic, gravity, heat flow and magneto-telluric (MT) data. (MT is an electromagnetic method used to map the spatial variation of the Earth's resistivity by measuring naturally occurring electric and magnetic fields at the Earth's surface).
This multi-disciplinary study revealed, for the first time, the presence of high density 'granulite' rocks just below the Deccan volcanic cover, the researchers claim. "This implies that almost entire thickness of the granitic-gneissic upper crust in this part of India has been eroded even before the eruption of Deccan volcanism took place," the authors say. "This inference strengthens our belief that this region may have been in a state of continuous uplift and erosion during pre-Deccan trap eruptive period and the uplift probably continues even now," they say. Such an evolutionary process can alone bring the earth's crust below the Deccan Traps to significantly shallower levels, the scientists report. This has been confirmed by borehole investigations in which a sediment thickness of only 6–8 metres was found between the crystalline basement rock (earth crust) and overlying Deccan volcanics.
What can cause such massive uplifts of the earth's crust? The researchers say their calculations indicate a high input of heat flow (29–36 milli Watts per square meter) from the mantle below both Latur as well as Koyna. "Such high input of heat from the mantle is unheard of," says Om Prakash Pandey, a heat flow expert and first author. "High input of subcrustal heat signifies that the overlying crustal column is subjected to a much higher temperature than comparable terrains elsewhere."
According the researchers, stresses caused due to ongoing uplift and high input of heat flow from the mantle is continuously accumulating in this part of the Earth's crust beneath the Deccan Traps within which earthquakes tend to nucleate. "Such localised stresses act over and above those generated by collision of Indian sub-continent with Eurasia, and thus "have added an entirely new dimension to our understanding of the occurrence of intraplate earthquakes within a stable continental region."